The present invention relates generally to the field of wireless digital communications and more particularly to a technique for encoding access channel signals.
The increasing use of wireless telephones and personal computers has led to a corresponding demand for advanced wireless communication services which were once thought only to be meant for use in specialized applications. In particular, wireless voice communication first became widely available at low cost through the cellular telephone network. The same has also become true for distributed computer networks, whereby low cost, high speed access to data networks is now available to the public through Internet Service Providers (ISPs). As a result of the widespread availability of both technologies, the general population now increasingly wishes to be able to access the Internet using portable computers and Personal Digital Assistants (PDAs) over wireless links.
The most recent generation of wireless communication technologies makes use of digital modulation techniques in order to allow multiple users to share access to the available frequency spectrum. These techniques purportedly increase system capacity for a radio channel of a given available radio bandwidth. The technique which has emerged as most popular within the United States is a type of Code Division Multiple Access (CDMA). With CDMA, each transmitted radio signal is first encoded with a pseudorandom (PN) code sequence at the transmitter. Each receiver includes equipment that performs a PN decoding function. The properties of the PN codes are such that signals encoded with different code sequences or even with different code phases can be separated from one another at the receiver. The CDMA codes thus permit signals to be transmitted on the same frequency and at the same time. Because PN codes in and of themselves do not provide perfect separation of the channels, certain systems have added an additional layer of coding, and/or use modified PN codes. These additional codes, referred to as orthogonal codes, and/or modified PN codes encode the user signals so that they are mathematically exclusive in order to further reduce interference between channels.
In order for the CDMA code properties to hold true at the receiver, certain other design considerations must be taken into account. One such consideration involves the signals traveling in a reverse link direction, that is, from a field unit back to the central base station. In particular, the orthogonal properties of the codes are mathematically optimized for a situation where individual signals arrive at the receiver with approximately the same power level. If they do not, interference between the individual signals which arrive at the base station increases. Precise control over the level of each signal transmitted on the reverse link is thus critical.
More particularly, most CDMA systems are structured such that the forward link channels, that is, the channels carrying information from the base station towards the field unit, are different from the reverse channels. The forward link typically consists of three types of logical channels known as the pilot, paging, and traffic channels. The pilot channel provides the field unit with timing and phase reference information. Specifically, the pilot channel contains a sequence of data bits that permits the field unit to synchronize its PN decoding function with the PN coding used in the base station. The pilot channel is, therefore, typically transmitted continuously by the base station to facilitate the field units demodulation of the other forward link channels.
The paging channel is used to inform the field unit of additional information needed to communicate. Such information is typically management information which informs the field unit of which traffic channels it may use, for example. Other types of paging messages are used to communicate system parameters, access parameters, neighbor lists and other information needed for the field unit to manage its communication in such a way that it does not interfere with other field units transmissions.
The forward traffic channels are used to transmit user data and/or voice signaling information from the base station to the field unit.
On the reverse link, there are typically at least two types of logical channels, including an access channel and traffic channels. The access channel is used by the field unit to send a message to request access to traffic channels when it has data to communicate to the base station. The field unit thus uses the access channel to make requests for connection originations and to respond to paging messages. The traffic channels on the reverse link serve the same purpose as the traffic channels on the forward link, namely, to transmit user data and/or digitized voice payload information.
Pilot channels are not typically used on the reverse link. There are perhaps several reasons for this. For example, the most widely deployed CDMA systems, such as the IS-95 compatible system as specified by the Telecommunications Industry Association (TIA), use asynchronous reverse link traffic channels. It is typically thought that the overhead associated with allowing each field unit to transmit on its own dedicated pilot channel is not necessary. It is also thought that the overhead associated with decoding and detecting a large number of pilot channels back at the base station would not justify any perceived increase in performance.